Example #2 shows Error "The layer geoportal.cuzk.cz/WFS_GN/WFService.aspx?SID=27082012_03561414729376000_PM_021892838861&SERVICE=WFS&VERSION=1.0.0&REQUEST=GetFeature&TYPENAME=gmgml:GEONAMES&SRSNAME=EPSG:102067 is not a valid layer and can not be added to the map"

Example #1 shows Error "capabilities document contained no layers."

I don't know, where are the differences between services. Could you help me?

Example #2 is a protected service. You have to add an additional parameter (in this case "sid") to the request for authorisation. The WFS 2.0 Plugin already supports standard HTTP-Auth (username, password). I will add support for this type of protection in one of the next releases.

Update:

The new version 0.8.2 is online.
Now you are able to use the protected WFS 2.0 (Geonames (Czech Republic))